The difference between DDR2 and DDR3

[certforumz]

On the appearance, both DDR3 and DDR2 use 240 pins and you can hardly see any differences between them. The only difference is the position of the key notch that prevents users from inserting the wrong type of RAM into their motherboards. The shift in the position of the key notch also means that DDR3 is not compatible with DDR2 and a separate DIMM slot has to be used.



Official JEDEC Specifications
                                DDR 2              DDR3
Transfer Speed    400 ? 800 Mbps    800 ? 1600 Mbps
Voltage          1.8V +/- 0.1V      1.5V +/- 0.075V
Internal Banks     4                       8
Prefetch             4bits              8 bits
Termination    Limited            All DQ signals
Topology            Conventional T    Fly-by
CAS Latency    3 ? 5               6 ? 10
Thermal Sensor    No                Yes (optional)

The official specification states that DDR speeds ran to DDR-400, DDR2 from 400 to 800, and DDR3 800 to 1600. Due to the fact that DDR2 800 and 1066 (an overclock version of DDR2 800) exist, DDR3 800 and 1066 will become an overlap speed with that of DDR2. As such, it will only be 1333 that is the first DDR3 speed to have better frequency/speed performance over the DDR2 RAM.

It is important to know that DDR, DDR2 and DDR3 are not compatible with each other.

The formula for calculating the data transfer rate on DDR2 is:
(memory clock rate) x 2 x 2 x 64 / 8

The formula for calculating the data transfer rate on DDR3 is:
(memory clock rate) x 4 x 2 x 64 / 8